<!DOCTYPE html>
<html lang="zh-CN">
<head>
    <meta charset="utf-8">
    <meta name="author" content="JZH">
    <script src="js/jquery-3.1.1.js"></script>
    <title></title>
    <style>
        *{
            padding: 0;
            margin: 0;
            box-sizing: border-box;
        }
        #content{
            height: 1500px;
            background-color: lightblue;
        }
        #overlay{
            width: 100%;
            height: 100%;
            background-color: rgba(0, 0, 0, 0.1);
            position: absolute;
            top: 0;
            left: 0;
            z-index: 999;
            display: none;
        }
        #alert{
            width: 400px;
            height: 200px;
            background-color: #fff;
            position: absolute;
            /*top: 100px;
            left: 100px;*/
            text-align: center;
            z-index: 1000;
            display: none;
        }
        #alert h2{
            margin: 0;
            height: 40px;
            line-height: 40px;
            font-size: 16px;
            background-color: #eee;
            text-align: center;
            cursor: move;
            /*禁止用户选择页面上的内容*/
            user-select: none;
            -webkit-user-select: none;
            -moz-user-select: none;
            -ms-user-select: none;
            position: relative;
        }
        #alert h2 .close{
            display: inline-block;
            width: 30px;
            height: 30px;
            position: absolute;
            top: 5px;
            right: 5px;
            line-height: 30px;
            text-align: center;
            transition: all 1s linear;
        }
        #alert h2 .close:hover{
            cursor: pointer;
            background-color: #ccc;
        }
        #alert form{
            width: 100%;
            height: 160px;
            padding-top: 20px;
        }
        #alert div{
            height: 40px;
            line-height: 40px;
        }
        #alert input{
            width: 260px;
            height: 30px;
            padding-left: 10px;
        }
        #alert .login{
            border: none;
            font-size: 16px;
        }
    </style>
</head>
<body>
    <!--内容区-->
    <div id="content"></div>
    <center><button type="button">来点我啊<tton></center>
    <!--遮罩层-->
    <div id="overlay"></div>
    <!--弹窗-->
    <div id="alert">
        <h2>
            <span>用户登录</span>
            <span class="close">&times;</span>
        </h2>
        <form action="javascript:;">
            <div><label>账号：<input type="text" placeholder="请输入账号"></label></div>
            <div><label>密码：<input type="text" placeholder="请输入密码"></label></div>
            <div><input class="login" type="submit" value="登录"></div>
        </form>
    </div>
</body>
<html>
<script>
$(function(){
    // console.log($('#alert').width()+' '+$('#alert').innerWidth()+' '+$('#alert').outerWidth());

    // 遮罩层的大小永远都是100%
    setOverlayWH();
    function setOverlayWH(){
        // 整个页面的大小 可视页面的大小+滚动条的大小
        var w = $(document).width();
        var h = $(document).height();
        // console.log('w='+w+'  h='+h);

        var w = $(window).width()+$(document).scrollLeft();
        var h = $(window).height()+$(document).scrollTop();
        // console.log('w='+w+'  h='+h);
        $('#overlay').width(w).height(h);
    }

    setAlertCenter();
    // 弹窗第一次出现居中，当浏览器窗口发生变化时，也要居中
    function setAlertCenter(){
        // top = (可视页面的高 - 弹窗的高)/2+滚动条的高    
        var left = ($(window).width() - $('#alert').width()) / 2 + $(document).scrollLeft();
        var top = ($(window).height() - $('#alert').height()) / 2 + $(document).scrollTop();
        $('#alert').css({left:left,top:top});
    }


    // 当页面尺寸发生变化或者发生滚动时，重新计算尺寸
    $(window).resize(function(){
        setOverlayWH();
        setAlertCenter();
    });
    $(window).scroll(function(){
        setOverlayWH();
        setAlertCenter();
    });

    $('button').click(function(){
        $('#overlay').show();
        $('#alert').show();
        // 禁用滚动条
        $('html').css('overflow','hidden');
        $('body').css('overflow','hidden');
    });
    // 关闭
    $('#alert .close').click(function(){
        $('#overlay').hide();
        $('#alert').hide();
        // 使用滚动条
        $('html').css('overflow','auto');
        $('body').css('overflow','auto');
    });

    // 让弹窗移动
    $('#alert h2').mousedown(function(ev){
        //当按下鼠标时,获取当前点击的位置距离元素左边和顶部的距离 
         var distanceX = ev.offsetX;
         var distanceY = ev.offsetY;

        $(document).mousemove(function(ev){
            //不管点击h2的哪个位置,始终获取鼠标相对于整个文档的偏移量 
            var left = ev.pageX - distanceX;
            var top = ev.pageY- distanceY;

            //左右不能越界
            if(left < 0){
                left = 0;
            } 
            if(left <= $(document).scrollLeft()){
              left = $(document).scrollTop();
            };
            if(left >= $(window).width()-$('#alert').width()+
            $(document).scrollLeft()){
                left = $(window).width()-$('#alert').width()+
                $(document).scrollLeft();
            }
            //上下不能越界
            if(top < 0){
                top = 0;
            } 
            if(top <= $(document).scrollTop()){
                top = $(document).scrollTop();
            };
            if(top >= $(window).height()-$('#alert').height()+
            $(document).scrollTop()){
                top = $(window).height()-$('#alert').height()+
                $(document).scrollTop();
            }
            $('#alert').css({left:left,top:top});
        });
    });
// 松开鼠标,取消移动 
   $(document).mouseup(function(ev){
       $(this).off('mousemove');
   });        
 
});
</script>